IHow can laser photons have the same precise energy?

The photons generated by a conventional quantum laser are all in the same quantum state. Doesn't that mean that they all have the same exact energy?

Yet, because of energy-time uncertainty, the exact energy of any particle can never be measured. Also, the Copenhagen Interpretation says that it is meaningless to talk about qualities of quantum objects that can never be observed.

Staff: Mentor

This is a separate issue from the issue raised in posts #3 and #5. Line width means that the photons emitted by any real laser are not perfectly in phase. But the laser photons being in a coherent state means that, even for an idealized laser that emits all its photons exactly in phase, the photons are not in an eigenstate of energy and so cannot be said to all have the same energy. All that can be said is that they all have exactly the same phase (in the idealized case).

Yet, because of energy-time uncertainty, the exact energy of any particle can never be measured. Also, the Copenhagen Interpretation says that it is meaningless to talk about qualities of quantum objects that can never be observed.

It may be observed, just never without some error or limit to measurement. All devices and all measurements will have noise sources.